Subscription email template updates

[shanbady]

What are the relevant tickets?

Closes https://github.com/mitodl/hq/issues/4890

Description (What does it do?)

This Pr updates the subscription email template according to https://github.com/mitodl/hq/issues/4890 figma

How can this be tested?

1. Checkout this branch
2. Subscribe to all the providers/units mitx, mitpe, ocw etc
3. We need to manually set the "created_on" date for a bunch of learning resources so that the daily email task picks them up. to do this, go into django shell and run the following:

import datetime
from learning_resources.models import LearningResource
from learning_resources_search.tasks import send_subscription_emails

new_date = datetime.datetime.now() - datetime.timedelta(hours=5)
for obj in LearningResource.objects.all()[:100]:
	obj.created_on = new_date
	obj.save()
	print(obj.resource_type)

4. next we need to configure email settings so we can send emails locally. Grab settings for email from rc and set the following in settings.py - EMAIL_HOST, EMAIL_HOST_PASSWORD ,EMAIL_HOST_USER ,DEFAULT_FROM_EMAIL ,EMAIL_SUPPORT ,EMAIL_PORT ,EMAIL_USE_TLS
5. restart celery to pick up settings changes docker compose restart celery
6. get into a django shell and manually trigger the daily subscription task:

from learning_resources_search.tasks import send_subscription_emails
send_subscription_emails("channel_subscription_type")

Additional Context

The email you receive during testing will have a broken logo image which is expected this gets deployed to a live environment.

Additionally, the links to unsubscribe and manage preferences will route to a currently non-existent page which is also expected until #1331 lands

[shanbady]

This PR is ready for review as-is however once #1331 lands I will need to resolve some overlapping changes. We also need #1349 to land so I can account for unpublished channels when sending emails

[abeglova]

👍